<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>canvas</title>
    <script type="text/javascript" src="js/jquery-3.2.1.min.js"></script>
    <style type="text/css">
        #canvas{
            width: 7rem;
            margin: .25rem 0 0 1.5rem;
            border: 1px solid black;
            display: block;
        }
    </style>
</head>
<body> 
    <canvas id="canvas" width="1000" height="600"></canvas>
</body>
</html>
<script type="text/javascript">
    /**
     * rem 布局初始化
     */
    $('html').css('font-size', $(window).width()/10);
    /**
     * 获取 canvas 画布
     * 获取 canvas 绘图上下文环境
     */
    var canvas = $('#canvas')[0];
    var cxt = canvas.getContext('2d');
    var balls = [];
    
    /**
     * 事件交互, 点击事件为例
     * isPointInPath(横坐标, 纵坐标)
     */
    for(var i = 0; i < 10; i++){
        var ball = {
            X: Math.random()*canvas.width,
            Y: Math.random()*canvas.height,
            R: Math.random()*50 + 20
        }
        balls[i] = ball;
    }
    
    draw();
    $("#canvas").click(function(){
        //标准的获取鼠标点击相对于canvas画布的坐标公式
        var x = event.pageX - canvas.getBoundingClientRect().left;
        var y = event.pageY - canvas.getBoundingClientRect().top;
        for(var i = 0; i < balls.length; i++){
            cxt.beginPath();
            cxt.arc(balls[i].X, balls[i].Y, balls[i].R, 0, Math.PI*2);
            if(cxt.isPointInPath(x, y)){
                cxt.fillStyle = "red";
                cxt.fill();
            }
        }
    });
    
    function draw(){
        cxt.fillStyle = "blue";
        for(var i = 0; i < balls.length; i++){
            cxt.beginPath();
            cxt.arc(balls[i].X, balls[i].Y, balls[i].R, 0, Math.PI*2);
            cxt.fill();
        }
    }
</script>